As is known, in the field of vehicle tire manufacture one of the more difficult objects to be reached has always been that of reducing the rolling resistance of the tire, achieving at the same time a good wear resistance and a satisfactory skid resistance on wet road.
It is also known that the difficulties for achieving the aforementioned object arise essentially from the fact that the tire rolling resistance on the one hand and the wear resistance and wet skid resistance on the other hand, are affected in an entirely opposite manner by the amount of reinforcing filler--substantially carbon black--employed in the composition used to manufacture the tire tread.
In order to reduce the rolling resistance of the tire, in fact, it would be desirable to reduce the amount of the reinforcing filler used in the composition (for instance, below 70 parts every 100 parts by weight of the composition polymer base): in so doing, however, a falling off to unacceptable values of both wet skid resistance and wear resistance of the tire has been observed.
In order to overcome somehow this limitation of the compositions comprising carbon black-based fillers, the prior art has suggested to partly or fully replace the latter by so-called "white" fillers, in particular silica, as is described for instance in European Patent application EP 0 501 227.
Even though silica-based reinforcing fillers have allowed to reduce the tire rolling resistance without significantly affecting wear resistance and wet skid resistance, their use is not devoid of drawbacks.
In fact, these reinforcing fillers, besides having a cost much higher than the cost of carbon black, have per se a poor affinity with the polymer base of the compositions employed in the manufacture of tires and--as such--require the use of suitable coupling agents that may chemically bind silica to the polymer matrix.
However, the need of using such coupling agents limits the maximum temperature attainable during the mixing and mechanical working operations of the composition, not to trigger off an irreversible thermal degradation of the coupling agent.
But the respect of the aforementioned temperature constraint involves a marked reduction in the very mechanical mixing action that is of the essential for an optimum dispersion of silica throughout the polymer matrix.
The ensuing insufficient dispersion of silica in the composition causes in its turn several drawbacks, basically related to the extreme variability and non homogeneity of the physico-mechanical characteristics of the composition from zone to zone of the same.
In the different attempts made to manufacture a tread starting from compositions including silica as main reinforcing filler, a remarkable drawing difficulty and a size variability of the tread obtained from the so-produced compositions have been observed.
To all these drawbacks another, non negligible one must be added, namely the reduced production capacity of the drawing apparatuses used for tread manufacture.
All this reduces the hourly production capacity of the plants, which causes an increase in production costs.
Lastly, the electric conductivity of tires including silica-based reinforcing fillers, is drastically reduced, which involves an accumulation of electrostatic charges that, in some cases, may disturb the electronic apparatuses which are aboard the vehicle or even cause sudden electric discharges of dangerous intensity.